Systems for reprofiling the eye for refractive correction have become extremely popular. Such systems typically employ a 193-nanometer (nm) argon-fluoride excimer laser, passing the light to the corneal tissue, where a very precise amount of tissue is “ablated” from the eye with a laser shot. A variety of delivery mechanisms are commercially used, including systems in which a fixed spot size is moved over the surface of the eye, in which the spot size is varied, and in which erodible masks are placed in the path of the excimer laser beam. In all of these systems, the ultimate goal is to change the profile of the corneal surface by volumetrically altering the amount of tissue within the cornea. Further, these techniques have been employed on the surface of the cornea underneath the epithelia using a technique for photorefractive keratectomy (PRK) as well as the intra-cornea technique known as LASIK, or laser in situ keratomileusis.
U.S. Pat. No. 5,376,086, issued to Khoobehi et al., discloses a laser surgical method of sculpting a patient's cornea that uses a mask system with multiple openings in which laser power transmission is controlled through the use of diffraction and absorption. Each hole in the mask acts like an individual light source, distributing laser power as a function of the hole's size, shape, and overlaid coatings. By summing the power output of each hole pattern over a given area, an average power distribution is generated. The technique, however, is limited because the power transmission of the mask is tailored only to a particular corneal surface by using topographical information of that surface. The corneal surface topographic data is the controlling mechanism for constructing the pattern of the mask itself. The surgeon can observe the topographic information and then pattern the mask according to that topographic information.
The present invention is directed to improving laser ablation of eye tissue that avoids or reduces shortcomings of previous methods.